Various types of machines are known for packaging various types of pourable food products, such as fruit juice, wine, tomato sauce, pasteurized or long-storage (UHT) milk, etc.
Such machines have different characteristics, depending on the type of package used, e.g. packages made of strip or sheet material, cups, bottles, tubs, etc.
One of the best-known packaging machines is the one marketed under the registered trademark Tetra Brik®—referred to purely by way of a non-limiting example—in which the packages or packs are formed from a continuous tube of packaging material defined by a longitudinally sealed web.
The packaging material has a multilayer structure comprising a layer of paper material covered on both sides with layers of heat-seal material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material comprises a layer of barrier material, e.g. aluminium foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material eventually defining the inner face of the package and therefore contacting the food product.
To produce aseptic packages, the web of packaging material is unwound off a reel and fed through a sterilizing unit, in which it is sterilized, for example, by immersion in a bath of liquid sterilizing agent, such as a concentrated solution of hydrogen peroxide and water.
Alternatively, or in addition to being treated with a liquid sterilizing agent, the web of packaging material may be treated by exposure to one or more sources of UV electromagnetic radiation, as described, for example, in European Patent Application EP-A-919246.
Downstream from the sterilizing unit, the web of packaging material is maintained in an aseptic chamber, in which it is dried, folded into a cylinder, and sealed longitudinally to form a continuous vertical tube. In other words, the tube of packaging material forms an extension of the aseptic chamber, is filled continuously with the pourable food product, and is then fed to a forming and (transverse) sealing unit for producing the individual packages, and in which the tube is gripped between pairs of jaws and sealed transversely to form aseptic pillow packs.
The pillow packs are separated by cutting the sealed portions in between, and are then fed to a final folding station where they are folded mechanically into the finished form.
Packaging machines of the type described above are used widely and satisfactorily in a wide range of food industries for producing aseptic packages from strip packaging material. Performance of the sterilizing unit, in particular, amply ensures conformance with regulations governing the sterility of the packages.
Within the industry, however, a demand exists for further improvement, particularly as regards the safety of UV devices, which may be used for both disinfecting and sterilizing various types of packaging material, such as strip and sheet material, cups, bottles, tubs, etc.
UV devices substantially comprise a UV radiation source housed in a casing and protected at the front by a screen made of material resistant and permeable to UV radiation. In commonly marketed devices, the screen is defined by a quartz plate.
Though perfectly suitable in terms of physical-chemical properties, quartz has various drawbacks. In particular, it is extremely expensive. Moreover, a quartz screen is fragile and, if broken, tends to form extremely hard, sharp fragments. In known machines, the UV device, and therefore the quartz screen, is so located as to be protected against impact, so that the risk of it breaking and leaving trace fragments of quartz on the packaging material is highly unlikely. Nevertheless, at present, the possibility cannot be entirely excluded. Though unlikely, breakage may be caused by anomalous vibration or thermal stress, by flaws in the structure of the material, by accidental forcing or impact during assembly, or by a combination of any of these.
Another drawback of quartz in this type of application is its tendency to dirty easily in normal operating conditions, and where it is extremely difficult, expensive, or even impossible to clean, whereas replacement is far from cheap.
Quartz also has the further drawback of any flaws, cracks, dirt, etc. locally affecting its optical properties, thus possibly resulting in uneven irradiation of the material being treated.